1. Field of the Invention
The invention relates to a probe for an oscilloscope.
2. Discussion of the Background
In the case of active probes for oscilloscopes, which measure ground-referenced or differential signals, the probe tip picking up the signal to be measured in a high-ohmic manner is connected to an amplifier acting as an impedance converter, of which the output provides a characteristic impedance generally of 50 ohms. The test signal is supplied from the output of this amplifier via a high-frequency cable to the input of the oscilloscope, which terminates the cable with its characteristic impedance, for example, of 50 ohms, in order to avoid reflections. Amplifiers of this kind generally provide a so-called direct-voltage offset error, that is to say, an input voltage of 0 volts does not correspond exactly to an output voltage of 0 volts. Moreover, amplifiers of this kind generally also provide amplification errors, that is to say, with a desired voltage amplification of, for example, one, a 1 volt change of the input voltage does not generate exactly 1 volt change of the output voltage.
In order to keep this error to a minimum, the use of complex amplifier topologies is known from U.S. Pat. No. 5,384,532. For example, in the case of a ground-referenced probe of broad bandwidth, the use of an amplifier, which is constructed as a composite amplifier consisting of a high-frequency path for high-frequencies and a low-frequency path for low frequencies and direct voltages, as an impedance converter, is known. The amplifier in the low-frequency path is an operational amplifier with the minimum possible offset. The high-frequency path for high-frequencies, for example, above 10 MHz, consists of a three-stage emitter-follower with transistors of the npn type and is designed as an AC-coupled amplifier with a blocking capacitor disposed between the probe tip and the amplifier input.
All of these complicated amplifier topologies are not adequate to keep offset errors and amplification errors sufficiently small over a given temperature range. The user must therefore interrupt the measurement before every measurement with the probe and after every temperature change outside a given interval, for example, of 5° C., and implement a manual calibration. If the user does not do so, or if this is not possible, for example, in the case of an automated, long-term measurement without user, false results must be anticipated.